Hitherto, in an air-conditioning apparatus such as a multi-air-conditioning system for a building, a refrigerant radiates or absorbs heat as a result of the refrigerant being circulated between an outdoor unit, or in other words heat source, disposed outside the building, and an indoor unit disposed inside the building. The air-conditioned space is thus cooled or heated by the heated or cooled air. In such a multi-air-conditioning system for a building, multiple indoor units are connected, and there is often a mixture of stopped indoor units and running indoor units. The refrigerant pipes that connect the outdoor units to the indoor units may also reach up to a maximum of 100 m. As the refrigerant pipes become longer, large quantities of refrigerant fill the refrigeration cycle.
The indoor units of such a multi-air-conditioning system for a building are typically disposed and used in indoor spaces where people are present (such as office spaces, rooms, or stores). At this point, if for some reason the refrigerant leaks out from an indoor unit disposed in an indoor space, the refrigerant poses a major problem from the perspective of its influence on the human body and safety, as a refrigerant may be combustible or toxic depending on type. In addition, even assuming that the refrigerant is not harmful to the human body, the refrigerant leakage is expected to lower the oxygen concentration in the indoor space and exert an adverse influence on the human body. Thus, technology configured to stop the system (stop compressor operation) when the refrigerant leaks out from the refrigeration cycle has been disclosed (see Patent Literature 1, for example).